40 BACTERIA IN RELATION TO PLANT DISEASES. 



important fact that an unusually large number of poor researches in this field obscured 

 the general view and covered the whole subject so to speak, with a wet blanket. 



THE CARRIERS OF INFECTION. 



This subject is now widely investigated and popular, particularly in relation to the 

 spread of human diseases, but when the first evidence was obtained showing that bacterial 

 diseases of plants are transmitted by insects almost nothing was known. 



The first exact experiments were by Merton B. Waite in 1891. That year he proved 

 conclusively that pear-blight is disseminated by bees in course of their visits to pear blos- 

 soms for nectar and pollen. 



In 1893, the writer obtained some evidence that the bacterial wilt of cucurbits is 

 transmitted by beetles, and some years later established the fact conclusively. 



In 1895, the writer obtained very typical cases of the bacterial brown rot of the potato 

 using the Colorado potato beetle as the agent of transmission. 



In 1897, the writer showed that the bacterial black rot of crucifers could be trans- 

 mitted by insect larvae (Plusia) and by molluscs (Agriolimax) and pointed out that there 

 was no evidence of transmission of the disease by wind. Brenner confirmed a part of this 

 and incriminated aphides. 



More recently gall-forming nematodes were observed by Hunger in Java (in 1901), 

 and by the writer in the United States (in 1908), to function as carriers of a bacterial 

 disease of tomato, tobacco, etc. 



In 1910, D. H. Jones, in Canada, proved apple-blight (Bacillus amylovorus) , to be dis- 

 seminated from diseased to healthy shoots by aphides and by bark-boring beetles 

 (Scolytus) . 



There is therefore every reason to believe that small animals play a large part in the 

 dissemination of these destructive diseases. Elsewhere full details are given. 



SPECIFIC DISEASES. 



Admitting the parasitism of bacteria in plants, are there any specific diseases? The 

 physician depends to a considerable extent on subjective symptoms for his diagnoses. 

 Headaches, pain in various organs, etc., give him many clues. He also has his clinical 

 thermometer. There is nothing in plants, however, so far as we know, corresponding to 

 the rise in temperature which we call fever. The plant pathologist must depend entirely 

 on objective signs — spots, stripes, distortions, enlargements, atrophy, yellowing, sudden 

 wilting, etc. Moreover, the plant body being much less highly organized than the body 

 of man and the domestic animals, one might expect less differentiation in objective signs 

 due to the action of various parasites and to a certain extent this is true. For instance the 

 soft-rot bacteria all produce much the same set of phenomena and are capable of attack- 

 ing plants belonging to widely separate groups. There are other organisms, however, that 

 seem to be restricted to particular families and the morbid phenomena which they origi- 

 nate can scarcely be mistaken for diseases due to any other micro-organism. Pear-blight 

 is a good example of such a disease. We know only one organism capable of causing this 

 train of phenomena. In like manner, so far as we know, only one organism is able to 

 cause the bacterial wilt of cucumber, only one is able to cause the olive-tubercle. These 

 three diseases are restricted so far as known, to as many families of plants, and there are 

 also restrictions within each family, not all genera or species being susceptible. In this 

 respect the causes of these three diseases are very different from the soft-rot organisms, 

 the action of many species of which overlap, e. g., we may have a soft-rot of the potato or 

 cucumber due to half a dozen different organisms, the signs being essentially the same. 

 From this point of view these, therefore, are the lowest type of bacterial parasites. 

 A third type of organism is able to produce quite specific over-growth phenomena in a 



